This invention concerns the preparation of (xe2x88x92)-norlabdane oxide, the full chemical name of which is 1,2,3a,4,5,5a,6,7,8,9,9a,9b-dodecahydro-3a,6,6,9a-tetramethylnaphtho-(2,1-b)-furan. For simplicity this material will generally be referred to herein as norlabdane oxide.
Norlabdane oxide may be structurally shown by structure (1) as follows: 
Norlabdane oxide is a well known fragrance material, which is widely used for providing ambergris-type odours to perfumes. Ambergris is a metabolic product of blue sperm whales which has been used in the past as a valuable constituent of fine fragrances. Natural ambergris itself is no longer used for this purpose. However, there is a demand for perfume ingredients with ambergris-type odours. Norlabdane oxide represents one of the preferred synthetic compounds with desirable ambergris-type odour and is commercially available under various trade names (notably as Amberlyn, Ambroxan, Ambrox or Amberoxide).
A number of synthetic procedures for norlabdane oxide have been published. Many of these procedures use naturally occurring (xe2x88x92)-sclareol, which may be structurally shown by structure (2) as follows: 
as the starting material from which norlabdane oxide is obtained in a multi-step synthesis.
U.S. Pat. Nos. 5,463,089 and 5,473,085 describe the conversion of sclareol by use of osmium tetroxide or ozonolysis to an epimeric mixture of methyl-ketone intermediates 12-acetyl-norlabdane oxide which may be structurally shown by structures (3a) and (3b) as follows: 
Structures (3a) and (3b) may both be represented by structure (3) as follows: 
The 12-acetyl norlabdane oxide of structure (3) is then converted by Baeyer-Villiger oxidation with m-chloroperbenzoic acid in sodium acetate buffer to an epimeric mixture of the acetates 12-acetoxy-norlabdane oxide which may be structurally shown by structures (4a) and (4b) as follows: 
Structures (4a) and (4b) may both be represented by structure (4) as follows: 
The 12-acetoxy-norlabdane oxide of structure 4 is then reduced to norlabdane oxide of structure (1) by use of LiAlH4/BF3.OEt2.
EP 0822191A describes a generally similar conversion of sclareol to norlabdane oxide, but in which sclareol is converted to 12-acetyl-norlabdane oxide via sclareol oxide. The structure of sclareol oxide may be shown by structure (5) as follows: 
Sclareol is oxidised with ozone, followed by treatment with alkaline hydrogen peroxide to give sclareol oxide. The sclareol oxide is then oxidised with an organic hydroperoxide, preferably tert-butyl hydroperoxide, to give 12-acetyl-norlabdane oxide. The 12-acetyl-norlabdane oxide is converted to 12-acetoxy-norlabdane oxide by oxidation with an organic peracid, preferably peracetic acid. The 12-acetoxy-norlabdane oxide is then reduced to norlabdane oxide with sodium borohydride in the presence of a transition metal salt. The reaction scheme of EP 0882191A may be represented as follows: 
The present invention is based on an alternative, novel approach to conversion of 12-acetoxy-norlabdane oxide to norlabdane oxide in place of the complex metal hydride reduction of the prior art as discussed above.
According to the invention there is provided a process for preparing (xe2x88x92)-norlabdane oxide from 12-acetoxy-norlabdane oxide, comprising subjecting 12-acetoxy-norlabdane oxide to a hydrogenation reaction to produce (xe2x88x92)-norlabdane oxide.
Compared with the complex metal hydride reduction of 12-acetoxy-norlabdane oxide of the prior art, use of a hydrogenation reaction can have the advantages of reduced effluent and lower cost.
The hydrogenation reaction is conveniently carried out using a hydrogenation catalyst, preferably a noble metal catalyst such as a platinum catalyst. Good results in terms of reactivity and selectivity have been obtained using a platinum/zirconium oxide catalyst, preferably 5% platinum/zirconium oxide catalyst.
The hydrogenation reaction may be carried out in any suitable solvent, particularly inert hydrocarbon solvents such as octane, hexane, decane, cyclohexane, etc, with cyclohexane currently being favoured.
The reaction may be shown as follows: 
The process of the invention may be carried out using one or both of the epimeric forms of 12-acetoxy-norlabdane oxide, ie pure epimer or an epimeric mixture.
The 12-acetoxy-norlabdane oxide is conveniently prepared as described in EP 0822191A, as discussed above. This involves conversion of sclareol oxide to 12-acetyl-norlabdane oxide by oxidation with an organic hydroperoxide, preferably tert-butyl hydroperoxide, followed by conversion of 12-acetyl-norlabdane oxide to 12-acetoxy-norlabdane oxide by oxidation with an organic peracid, preferably peracetic acid. This process produces the 12-acetoxy-norlabdane oxide in the form of an epimeric mixture, which may be used without requiring purification.
The 12-acetoxy-norlabdane oxide may alternatively be prepared by other known techniques, for example as described in U.S. Pat. Nos. 5,463,089 and 5,473,085.